Conventionally, a pressure-accumulating fuel injection system is known that injects high-pressure fuel accumulated in a common rail into a cylinder of an internal combustion engine (refer to U.S. Pat. No. 6,895,936 and its counterpart JP-2005-36794-A). The pressure-accumulating fuel injection system is provided with a feed pump that pumps up fuel from a fuel tank and supplies the fuel to a high-pressure pump, and a regulation valve that regulates a feed pressure of the feed pump (a fuel pressure of the fuel supplied from the feed pump to the high-pressure pump).
As shown in FIG. 15A, the regulation valve is composed of a cylinder-shaped valve body 110 that has an inlet port at one end side, a piston 120 that is reciprocatably inserted inside the valve body 110, a spring 130 that is disposed behind the piston 120, and so on.
The valve body 110 is provided with relief ports 140 and an oil drain port 150 that open on a sidewall, and an oil accumulation chamber 160 that is formed behind the piston 120 and filled with fuel. The fuel in the oil accumulation chamber 160 enters and exits through the oil drain port 150 in accordance with a motion of the piston 120.
The regulation valve regulates the fuel pressure applied to the piston 120 (the feed pressure of the feed pump) to a specific value by matching the piston 120 at a position to balance the fuel pressure entering from the inlet port 100 and a summation of the fuel pressure in the oil accumulation chamber 160 and a reaction force of the spring 130. As shown in FIG. 15B, a plurality of the relief ports 140 are shaped round to have diameters equal to each other, spaced at regular intervals in a circumferential direction of the valve body 110, and placed at the same position in a moving direction (horizontal direction in the figure) of the piston 120.
Thus, the opening area of the relief port 140 in accordance with a displacement of the piston 120 varies as an opening area property shown in FIG. 16. A point O in the figure represents a piston stable position at which the motion of the piston 120 is stabilized. A1 in the figure represents a distance from the piston stable position to a valve-opening position (a position in which the piston 120 totally closes the relied port 140 in moving in a direction to close the relief port 140, or a position in which the piston 120 starts opening the relief port 140 in moving in a direction to open the relief port 140).
However, in the above-described regulation valve, when air is trapped in the fuel sucked by the feed pump, the fuel pressure applied to the piston 120 becomes unstable. Then, as shown in FIG. 5A, the piston 120 vibrates to repeat an opening and closing behavior (a behavior to open and close the relief port 140) periodically, so that a valve opening and closing noise (a groaning unusual noise) occurs. The valve opening and closing noise grates on humans' ears to cause a driver's discomfort, so that measures against the unusual noise is desired.